Tire Size Comparison Calculator
Compare nominal tire diameter, sidewall, width and circumference. See how a diameter change affects estimated speed.
Where do I find these specs?
Read the tire size on its sidewall. For a wheel marked 7.5J × 17 ET40, enter a width of 7.5 in and offset of 40 mm; 17 is the rim diameter in the tire size. Check wheel markings or product documentation. Include any existing spacer in the current effective offset.
What changes
+2.4 mm taller
Position changes are calculated; fitment is not confirmed. Check actual tire dimensions and clearance through steering and suspension movement. Calculation assumptions ↗
Tire-size comparison
Both setups use the same scale. The schematic shows nominal diameter, rim diameter, sidewall height and section width. Tread and contours are illustrative; actual tire dimensions vary.
Detailed measurements
| Measurement | Existing | New | Change |
|---|---|---|---|
| Overall diameter | 631.9 mm | 634.3 mm | +2.4 mm |
| Section width | 205.0 mm | 225.0 mm | +20.0 mm |
| Sidewall height | 112.8 mm | 101.3 mm | -11.5 mm |
| Circumference | 1985.2 mm | 1992.7 mm | +7.5 mm |
| Revolutions / mile | 811 | 808 | -3 |
How to compare tire sizes
Enter the marking on your current tire and the proposed size. For 205/55R16, 205 is the nominal section width in millimeters, 55 is the sidewall height as a percentage of that width, and 16 is the rim diameter in inches. Compare diameter, width and sidewall together; a larger rim can have a similar overall tire diameter.
Worked example: 205/55R16 vs 225/45R17
The nominal diameter changes from 631.9 mm to 634.3 mm: an increase of 2.4 mm (0.38%). Section width increases by 20 mm, while each sidewall is 11.5 mm shorter. These are calculated dimensions, not measurements of a particular tire model.
Does a similar diameter mean the tire will fit?
No. Check the tire manufacturer’s approved rim widths and measured dimensions, the vehicle’s load and speed requirements, and clearance through steering and suspension movement. This calculator does not approve a replacement size.
Why can two tires with the same marking measure differently?
The marking describes a nominal size. Tire construction, the measuring rim, wear and operating conditions affect actual dimensions and rolling circumference. Use the specific product’s published measurements when checking clearance.
How do I compare the wheel position as well?
Use the wheel-position comparison with wheel width and offset, or the wheel offset calculator. Tire width alone does not tell you how far a wheel moves inward or outward.
Formula reference: Tire Rack’s tire dimension guide. For speed changes, use the speedometer calculator.